Liver is composed of two major cell types: the tissue parenchyma or hepatocytes, and reticuloendothelial (RE) cells. RE cells make up about 30% of the total cell population in liver; however, little is known about their metabolism is relation to liver function or in relation to the complete reticuloendothelial system (RES). Hepatic RE cells account for a major portion of the total phagocytic capacity and therefore play an important part in immunological resistances and in the clearance of effete or damaged cells from blood. Several procedures described in the literature for isolating RE cells from liver were evaluated, and our conclusion is that these give very inhomogeneous populations of RE cells in low and variable yields. We have perfected an alternative method for isolating intact RE (Kupffer) cells from rat liver. Cells are isolated by labeling them with colloidal iron, digestive perfusion of liver, tissue dispersion, filtrations, and separation of the iron-loaded cells in a magnetic field. This method gives high yields of 99% pure hepatocytes and 99% pure iron-loaded RE cells from a single rat liver. We now propose a project for the biochemical study of cells isolated by iron loading and developmnt of new isolation techniques for hepatic RE cells. This research includes: (1) continued evaluation of the iron-loading technique for isolating Kupffer cells, (2) isolation of hepatic RE cells by digestive perfusion and Ficoll gradient centrifugation, (3) biochemical study of isolated Kupffer cells and other hepatic RE cells. This will include a qualitative and quantitative study of enzymes isolated from RE cells, with emphasis on enzymes which might enable these cells to recognize elements to be phagocytized and others which give these cells the ability to digest and/or detoxify foreign bodies. Enzymes from liver RE cells will be compared with similar enzymes from hepatocytes and other macrophages.